Field of the Invention
The present invention relates to systems for cooling which employ an evaporator. More specifically, the present invention relates to a refrigeration system employing a control scheme to manage the flow of foam into the compressor.
Refrigeration systems of today, up to about 15 tons, control the flow into the evaporator such that much evaporator surface is not wasted due to needless superheating of the vapor exiting the evaporator. Thermal expansion valves are widely used to control superheat. A liquid filled bulb measures the temperature of the refrigerant leaving the evaporator. This temperature generates a pressure that is communicated to a first side of a diaphragm in the expansion valve. A second side of the diaphragm is exposed to the evaporating pressure. If the temperature gets too high leaving the evaporator, a higher pressure on the first side of the diaphragm drives the valve open further, admits more refrigerant resulting in a lower exiting vapor temperature. The evaporating pressure also rises somewhat due to more effective use of the evaporator surface. The overall response of the valve controls the superheat but wastes surface by superheating.
Other systems utilize an electrically heated thermistor at the outlet of a direct expansion evaporator in order to control the flow of refrigerant into the evaporator. The heated thermistor valve controls flow into the evaporator because the thermistor temperature drops very quickly when liquid refrigerant impinges on it. It controls close to saturation which is ideal for the evaporator heat transfer. However, this valve is more expensive than the old thermal expansion valves described herein above and has precluded its widespread use up to the present day.
In systems of the size range of interest herein, still other systems utilize an electronic expansion valve which responds to liquid level in the evaporator shell. If the level drops, the valve is opened slightly; whereas if it rises too high, the valve is closed slightly. This is good for a conventional chiller but would not be satisfactory for a system designed to ingest foam from the evaporator.
With the ingested system, the level must be higher under low load (capacity) because less flow is available to generate and suck foam. Under high load, the level must be somewhat lower or too much liquid will be ingested into the compressor.
In an ingestion process it is advantageous to admit enough liquid into the compressor from the evaporator to seal, cool, lubricate, and quiet the compression process itself. The liquid that is ingested from the evaporator will be mostly liquid refrigerant but, there may be some percentage of lubricant as it is desirable to have at least a tiny amount of lubricant dissolved in the liquid refrigerant. Pure liquid refrigerant is not a viable lubricant for metallic interfaces. It is an excellent coolant but does not have an effective quality of lubrication. Its viscosity is about one third that of water and water is not considered a good lubricant. In any event, it appears desirable to have a tiny amount of lubricating material contained in the liquid refrigerant. One problem is that the lubricant and refrigerant mixture foams within the evaporator. What is needed is a control scheme which reacts quickly to changing load conditions for use with an ingestion system.